The influence of bicarbonate concentration and ionic strength on peroxide speciation and overall reactivity towards UO2

Abstract

H₂O₂ produced from water radiolysis is expected to play a significant role in radiation induced oxidative dissolution of spent nuclear fuel under the anoxic conditions of a deep geological repository if the safety-barriers fail and ground water reaches the fuel. It was recently found that the coordination chemistry between U(VI), HCO₃²⁻ and H₂O₂ can significantly suppress H₂O₂ induced dissolution of UO₂ in 10 mM bicarbonate. This was attributed to the much lower reactivity of the U(VI)O₂²⁺-coordinated O₂²⁻ as compared to free H₂O₂. We have extended the study to lower bicarbonate concentrations and explored the impact of ionic strength to elucidate the rationale for the low reactivity of complexed H₂O₂. The experimental results clearly show that dissolution of U(VI) becomes suppressed at [HCO₃⁻] < 10 mM. Furthermore, we found that the reactivity of the peroxide in solutions containing U(VI) becomes increasingly more suppressed at lower carbonate concentration. The suppression is not influenced by the ionic strength, which implies that the low reactivity of O₂²⁻ in ternary uranyl-peroxo-carbonato complexes is not caused by electrostatic repulsion between the negatively charged complex and the negatively charged UO₂-surface as we previously hypothesized. Instead, the suppressed reactivity is suggested to be attributed to inherently higher stability of the peroxide functionality as a ligand to UO₂²⁺ compared to as free H₂O₂.